#include <iostream>
#include <queue>
#include <cstring>

using namespace std;

const int N = 100010;
int h[N], ne[N], w[N], e[N];
int idx, n, m;
int dist[N];
bool st[N];

void add(int a, int b, int c)
{
    e[idx] = b;
    w[idx] = c;
    ne[idx] = h[a];
    h[a] = idx++;
}

int spfa()
{
    memset(dist, 0x3f, sizeof(dist));

    queue<int> q;
    dist[1] = 0;
    q.push(1);
    st[1] = true;

    while (q.size())
    {
        int t = q.front();
        q.pop();
        st[t] = false;
        for (int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if (dist[j] > dist[t] + w[i])
            {
                dist[j] = dist[t] + w[i];
                if (!st[j])
                {
                    st[j] = true;
                    q.push(j);
                }
            }
        }
    }
    return dist[n];
}

int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof(h));
    while (m--)
    {
        int a, b, c;
        scanf("%d %d %d", &a, &b, &c);
        add(a, b, c);
    }
    int t = spfa();

    if (t == 0x3f3f3f3f)
    {
        printf("impossible");
    }
    else 
    {
        printf("%d", t);
    }
    return 0;
}